With a spring of this type, one of the difficulties encountered is that the spring has to be fitted within the housing defined in the torsion damper for containing the spring (this housing being necessarily curved), at least without the occurrence of any friction not serving a useful purpose. In this connection, the spring can tend to make radial contact between the ends of the spring, in an uncontrolled way and such as to give rise to unwanted parasitic friction, with one or both of the two coaxial parts of the torsion damper.
A first solution which is known in the art consists in shaping the spring so as to match its housing, by preforming it with a curvature similar to that of the housing. However, the process of shaping the spring in this way, which involves either increasing the separation of its various turns on the radially outer side, one by one, or directly hot-forming the spring as a whole, is tiresome and costly.
A second known solution consists in dividing the spring into distinct portions of its length, by interposing between these predetermined portions of the spring spacing pads which are of suitable wedge shapes. The spring then becomes effectively a multiplicity of straight springs, connected in series in accordance with a generally polygonal profile, instead of a single curved spring having a circular profile. With this arrangement, apart from the fact that these intermediate spacing pads between the straight springs are additional components, which adds to the cost, it is necessary to cut or machine one turn of each of the straight springs, at each end of the latter, so as to provide a suitable thrust surface for the spring to bear on the spacing pads.
The general result of this is that, other conditions being equal, the efficiency of the arrangement is reduced to a degree depending on the total number of turns which thus have to be cut, and which are therefore made thinner or even "dead". This effect is inevitable in a spring which is arranged in this manner.